Conductive bonding design and method for aluminum backed circuits

ABSTRACT

The present invention provides a new device and technique for enhancing the electrical properties of the thick metal backer/adhesive bond/ground plane interface. The enhanced electrical properties are obtained by micro-roughening a connection surface of the thick metal backer prior to forming the thick metal backer/adhesive bond/ground plane interface.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 08/855,812filed on May 12, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to a device and method for forming animproved conductive interconnection between a voltage plane on the backside of a printed circuit card and a thick metal heat sink.

Many modern high power radio frequency or microwave applications requiresignificant current carrying capability and/or significant thermaldissipation. To meet these needs, a thick metal backer (“TMB”) is oftenconnected to an external ground plane located on the surface of aprinted circuit board (“PCB”) to improve the electrical performance ofthe ground plane and also to provide a heat sink for thermaldissipation. Current methods for connecting such a thick metal backplate include sweat soldering, non-conductive adhesive bonding followedby plating through holes for electrical interconnection, gold coatingthe ground plane and applying a silicone adhesive which includes amixture of silver particles, or by mechanical interconnection such asscrews, rivets or soldered pins. Each of these methods have proven lessthan ideal as they tend to be too costly while compromising performance.Also, some of these methods result in poor reliability in the resultingcircuit board assembly.

The sweat solder method entails soldering a conductive ground planelocated on the back of a PCB onto a TMB. The resulting electricalperformance is acceptable, because a highly conductive interface isformed. This interface allows for conductive interconnection across theentire ground plane. The reliability is less than ideal, however, due tothe low compliance of the soldered interconnection. This low complianceresults in the interconnection being unable to accommodate mechanicalstress caused by the mismatch of the coefficient of thermal expansion ofthe PCB compared to that of the ground plane. Also, such sweat solderingis a complex and costly process prone to defects. Defects can occur inthe bonding because the bond may reflow during subsequent processing.

Non-conductive adhesive or fusion bonding of the PCB to the TMB also isnot ideal due to prohibitive cost, unreliable performance and complexmanufacturing. Often, raw dielectric materials, for examplepolytetrafluoroethylene microwave laminates, are pre-bonded to a TMB ofcopper, brass or aluminum. The TMB is then discretely interconnectedwith the PCB using plated through holes or mechanical connecting meanssuch as pressure-fitted or soldered pins. Such discrete electricalinterconnects are less desirable than an area electrical interconnectiondue to compromised localized grounding.

Simple non-adhesive mechanical interconnection such as screws or rivetsmay also be used to join the PCB and TMB together but imperfectcoplanarity between the PCB and TMB, coupled with localized thermalstresses tends to induce localized areas of non-contact, preventingglobal interconnection and changing the contact resistance over time.Also, resistance stability across the interface is less than desirableand ultimately may lead to failure of the device.

The mechanical and electrical stabilities of conductively-bondedinterfaces between the ground plane on a PCB and the TMB have provendifficult to control. Typically, a TMB is formed of copper, brass oraluminum depending upon the need for electrical and thermalconductivity, weight, and ease of machining versus economicconsiderations. The bonding surface on the PCB is often provided with acopper, tin-lead, tin or gold finish. The TMB is typically aluminum,brass or copper. With the exception of gold, these metals are prone-tooxidative, hydrolytic and corrosive processes. The processes lead to theformation of metal oxides, hydroxides and other corrosive products atthe interface between the conductive adhesive and metal adherend whichultimately can compromise both the electrical and mechanical stabilityof the bonding and eventually, the performance and reliability of thepackaging structure. This problem is particularly troublesome in humidenvironments, especially in the case of adhesively bonded aluminum.

Aluminum surfaces are normally protected by a thin layer of aluminumoxide that provides passivation of the metal at room temperature andmoderate relative humidity. Although the native oxide of aluminum is apoor conductor, it is thin enough to allow a reasonably low contactresistance for conductive interconnections and typically resists furtherdegradation of the electrical interconnection. However, experience hasshown that PCB's conductively bonded to aluminum and exposed to highervalues of temperature and humidity induce a transformation of thealuminum oxide (Al₂O₃)to aluminum oxyhydroxide (AlOOH) and finally, ifthe reaction is complete, aluminum hydroxide (Al(OH)₃). Aluminumhydroxide is mechanically weak, non-conductive and non-passivating(i.e., offers no further protection of the underlying aluminum/aluminumoxide from corrosion). This aluminum hydroxide can build up tosignificant thicknesses which can result in significant increases ininterfacial resistance through the bond and to mechanical separation ofthe bonded surface. The rate of formation of the aluminum hydroxide ismuch greater under a conductive bonding adhesive, such as silver-filledepoxy, than for exposed aluminum, perhaps due to corrosive species inthe adhesive resin, and/or to galvanic coupling between the silver inthe adhesive and aluminum.

Roughening of surfaces by sandblasting, chemical etching or anodizationhas been commonly practiced to enhance adhesion of aluminum/polymeradhesive systems and to provide structural durability in humid orcorrosive environments. However, when the bond itself must beelectrically conductive, treatments that improve adhesion by producing athick oxide layer, such as phosphoric acid or chromic acid anodization,are not suitable because of the poor electrical properties of the thickoxide layers. Sand blasted surfaces show some improvement in bondresistance stability, but still significant susceptability to hydrolysisand resistance increase in humid environments.

Accordingly, there is a need for new technology for fabricating reliableconductive interconnections between metal substrates, particularlyaluminum substrates, and ground planes of printed circuit boards,especially when the resulting circuit board assembly is subject to humidenvironments over extended periods of time.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been found thatmicro-roughening a connection surface of a thick metal backer precedingapplication of an electrically conductive adhesive can reduce or eveneliminate the increase in electrical resistance of a metal/adhesive bondwhen subjected to humid conditions.

Thus, the present invention provides a new design and technique forenhancing the electrical properties of the thick metal back/adhesivebond/ground plane interface obtained by micro-roughening the surface ofthe TMB. The technique comprises micro-roughening a connection surfaceof the TMB with an aqueous slurry applied under pressure to obtain arelatively uniform roughness and oxide layer across the connectionsurface of the TMB. The connection surface is then adhered to apre-treated ground plane of the PCB with preferably an electricallyconductive adhesive. The joining of the TMB to the ground plane isperformed under pressure with the adhesive being cured to form a stableconductive bond.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are a graphical representation of the results obtained inExample 1 discussed later in this specification.

DETAILED DESCRIPTION

Conductive bonding of thick metal backers or plates onto a ground planeof a printed circuit board can be accomplished using a variety ofconductive adhesives and for a variety of applications. The presentinvention provides an improved conductive interconnection between theTMB and the PCB by micro-roughening the connection surface of the TMBbefore adhering together the TMB and PCB. In particular, thismicro-roughening is best accomplished by a pressurized vapor-gritblasting to develop the roughened connection surface.

The surface of the ground plane that is to be bonded to the connectionsurface of the TMB can be of a metal such as copper, gold,palladium-nickel, tin-lead or tin. If copper is utilized for the groundplane, an oxide inhibiting treatment such as benzotriazole isbeneficial, after an operation to remove surface oxide, in order tomaintain low interfacial electrical resistance and good bond strength.Stability of electrical resistance and optimum bond strength anddurability is obtained in a copper ground plane coated with tin. The tinmay be coated over the copper ground plan by electroless, immersion orelectrolytic deposit. The thickness of the coating can range from 20^(μ)in to 250^(μ) in or thicker, with optimum results occurring around150^(μ) in. Furthermore, improved bond resistance stability anddurability is obtained by cleaning the tin prior to bonding, with acommercial tin conditioning solution such as PD510, manufactured byMacDermid. (Preferably containing a strong complexant for copper such asThiourea).

Once the ground plane surface has been prepared for bonding, theconnection surface of the TMB needs to be readied for adhering the TMBto the ground plane. While typical metals used for a TMB include copper,brass and aluminum, it is preferable to use aluminum for the TMB.However, it is contemplated that other metals could be used for the TMBof the present invention.

To prepare the connection surface of the TMB for microroughening, theconnection surface is preferably first degreased. Isopropyl alcohol inan ultrasonic bath or accompanied by mechanical scrubbing has beendemonstrated to be an effective degreasing treatment. It is contemplatedthat other such procedures using commercially available cleaners andsolvents, such as acetone, are also acceptable. Following thisdegreasing, the connection surface of the TMB is micro-roughened.

The connection surface is vapor-grit blasted in order to achieve a TMBwith a relatively uniform micro-roughened surface that has a relativelythin oxide layer. Roughness and total surface area of the connectionsurface appear to be important factors in bond integrity with aroughness in the range of 20^(μ) in to 80^(μ) in being suitable. Whilevarious methods exist for micro-roughening the connection surface, vaporblasting is preferred and yields the optimum results. Vapor blasting canbe defined as spraying under pressure a fine grit abrasive suspended ina medium onto a surface using a spray processing tool which can beconveyorized. Preferably, the medium is aqueous. One such conveyorizedspray processing device is a Liquid Honing Vapor Blast. Baselineconditions for this conveyorized spray processing device are about 70 to80 psi spray pressure and 1 meter/minute conveyor speed. The spraypressure, however, may be in the range of about 65 to 150 psi whileyielding the desired micro-roughened connection surface. Optimal resultsare obtained with a spray pressure of about 90 psi. The preferredabrasive materials are aluminum oxide or titanium oxide, in the form ofa 320 mesh, present in an aqueous slurry at a concentration of about12%-16% solids by weight. After vapor blasting, the connection surfaceshould have a roughness in the range of about 30^(μ) in to about 70^(μ)in and a resulting oxide layer of 5-10 nm. Roughness is defined as theaverage peak to valley height taken across a sample area of the TMBconnection surface.

The resulting micro-roughened connection surface of the TMB results in areduction in resistance drift under conditions of high temperature andhumidity. For example, under 85° C. and 80% relative humidity, reductionin resistance drift of a magnitude of 10× to 100× can be obtained over a1000 hour time period for an aluminum TMB bonded to a PCB.

A wide variety of adhesives can be used to bond the TMB to the groundplane of the PCB. In one embodiment of the present invention, theadhesive is electrically conductive. These adhesives include, but arenot limited to, isotropically conductive epoxies such as highly loadedsilver-flake filled epoxies, such as, Ablestik 8175, anisotropicallyconductive epoxies, for example, epoxies loaded with silver platednickel particles, silver or gold glass spheres, or thermoplastic-basedconductive adhesives. Preferably, the present invention utilizesconductive epoxy materials in screenable paste form such as Ablestik8175. Also, if the PCB and TMB are similar in their coefficient ofthermal expansion (“CTE”), it is preferable for the adhesive to beclosely matched in CTE to both the PCB and TMB. If the PCB and TMBdiffer substantially in CTE, it is preferable for the CTE of theadhesive to be intermediate to those of the PCB and TMB. Also, in oneembodiment, the adhesive is an epoxy thermoset, but it is contemplatedthat other adhesives such as epoxy thermoplastics could be utilized.

The adhesive can be applied to either the surface of the PCB, theconnection surface of the TMB or to both surfaces. Best results areobtained when the adhesive is applied to the connection surface of theTMB. Furthermore, the preferred method of applying the adhesive is ascreening process in order to achieve coverage over the large area whichis typical in card bonding applications. Other methods such asstenciling, however, are acceptable.

Thus, to produce the printed circuit board assemblies with thick metalback, the connection surface of the TMB is preferably degreased and thenmicro-roughened by a vapor-grit blast. Also, the surface ground plane ofthe PCB is preferably cleaned and oxide layers removed. Oxidationinhibiting treatments may also be beneficial. Once these pre-connectiontreatments are completed the electrically conductive adhesive isdispensed and the PCB and TMB are clamped together under pressure andthen heated to cure the adhesive, resulting in an aluminum backedcircuit with a conductive bond in which the TMB acts as a heat sink andshield plane or current-carrying plane.

The time between the micro-roughening treatment and the application ofthe adhesive should be kept to a minimum, and the bonding and curingoperations should be performed soon after adhesive screening. Hold timesare preferably less than four hours, but around eight hours is oftenacceptable. During hold time, however, environmental control oftemperature and humidity is desirable to minimize degradation of thebonding surfaces. Also, clearance areas around the edges of the bondedareas and recessing of the adhesive from the edges of the PCB or TMB areoften necessary to prevent excessive dispersment of adhesive duringbonding and curing. Once the TMB and PCB have been treated and adhesiveapplied to one or both of the connection surface and the ground plane,the connection surface of the TMB is joined to the ground plane underpressure to form a TMB/adhesive bond/ground plane interface of a circuitboard assembly. Also, once joined, the circuit board assembly is curedso that the adhesive forms a stable interconnection between the TMB andthe PCB. Proper alignment of the PCB to the TMB is important and can beaccomplished by mechanical means such as use of pins in common toolingholes, edge location, or optical registration.

The mating of the PCB to the TMB should be performed in a manner thatminimizes entrapment of air in the bond. Air trapped in the bond canalter the electrical performance of a metal backed circuit especially inapplications where localized grounding is required and critical. Also,trapped air can reduce the reliability of the bond and provide a conduitfor the ingress of moisture resulting in accelerated hydrolysis ofaluminum in the bonded areas. When the adhesive is applied to the TMB byscreening, any air trapped when the PCB is mated to the TMB is generallyfound between the bonding surface on the board and the adhesive.

While numerous means exist to minimize entrapment of air, the mosteffective means of evacuating and minimizing entrapped air involve theuse of vacuums. In one vacuum method, the assembly operation is carriedout in a vacuum chamber. The PCB is aligned and mated to the TMB andthen clamped in place while in the chamber. A vacuum lamination presscan also be utilized. The minimization of air entrapment is bestachieved by applying the vacuum before the application of pressure. Thiscan be successfully accomplished in a system where the card is placed onthe backer with no pressure other than the weight of the card, theassembly is placed under vacuum and pressure is then applied byinflating a bladder which applies pressure centrally first and thenradiating outward toward the edges of the assembly.

The bonding and curing process is tailored to a given adhesive systemwith pressure being applied uniformly. The bonding pressure often isdeterminative of the mechanical durability and electrical stability ofthe conductive bond. The appropriate bonding pressure is related to theadhesive being used. For example, when the adhesive is an unstabilizedsilver flake-filled epoxy based system in paste form, very low pressureis optimal. Pressure required for these adhesives is in the range ofabout 0.8 to about 1.5 psi However, anisotropic conductive epoxies andconductive thermoplastic adhesives frequently require substantially morepressure. For these adhesives, pressure at 20 to 50 psi or higher wouldbe common.

Adhesives requiring a higher pressure require bonding tooling such asflatbed or autoclave lamination press. Isotropically conductiveadhesives that require low bonding pressure, however, can be bondedusing a low pressure clamping device or a dead weight. While pressureapplied is a function of the adhesive used, no matter what adhesive ischosen, the pressure should preferably be applied uniformly. Pressureuniformity is ensured by using an appropriate compliant press-padmaterial between the TMB and the pressure fixture with pressure suppliedby a spring-loaded clamp or a weight. Best results have been achievedwith a ⅛″ rubber pad which helps eliminate voiding in the adhesive. Withlow pressure bonding processes, special care must be taken to eliminateentrapped air in the bond line. Methods include the previously mentioneduse of a vacuum and applying pressure from the center and radiatingoutward using a bladder. Another method involves using a rolling deviceto force air out of the bond line. This rolling process could beperformed manually, but would preferably be automated to controlpressure and uniformity of the applied pressure.

Once the TMB and PCB have been joined, the rubber pad is placed withinthe clamp fixture so the TMB, PCB and pad are all placed together intothe clamp. The clamp is closed to apply pressure, and the adhesive iscured by heating. The use of a small free-standing clamp enables curingof the adhesive in a conveyorized IR oven. This allows a high bondingprocess throughput and a rapid heating ramp-up rate. Rapid heating ispreferable because slow heating allows excessive bleed out of theadhesive which can interfere with component assembly surfaces and canincrease resistance instability of the bond. The preferred temperatureramp rate for the Ablestick 8175 adhesive is about 10 to about 12°C./min to cure temperature of about 130 to about 165° C. for a cure timeof about 25 to about 45 minutes. A preferred cure temperature has beenfound to be about 150° C.

In order to more thoroughly illustrate the present invention, thefollowing example was constructed. The electrical properties of the bondformed between the connection surface of an aluminum TMB and thetin-plated copper ground plane of the PCB were tested by measuringelectrical resistance as a function of time. The TMB and PCB were bondedtogether according to the above explained invention. The electricalproperties were tested by placing the circuit board assembly in achamber maintained at 85° C. and 80% relative humidity. Resistance wasmeasured for periods of up to 800 hours to determine the effect of highhumidity conditions on the electrical properties of the invention. Also,the experiment in each example was repeated a number of times to insurereliability, and in some instances testing was confirmed for over 4000hours.

EXAMPLE 1

In this example, one set of circuit board assemblies (“sample 1”) wasformed with a vapor blast treatment of the aluminum connection surfaceprior to bonding while a second set of circuit board assemblies (“sample2”) was formed without the vapor blast treatment. The comparison of theelectrical properties of the two sample populations is illustrated inFIGS. 1 and 2. Sample 1 was formed with an electroplated tin groundplane. The TMB was degreased and grit-blasted at about 90 PSI yieldingan average peak to valley roughness of 30 to 70^(μ) in. The TMB wasattached to the ground plane of sample 1 with Ablestick 8175, placed ina clamping fixture and cured for 50 minutes at a peak cure temperatureof 180° C. down to 150° C. Sample 2 was prepared in the same manner assample 1 except that the TMB was not vapor grit blasted prior tobonding. Once the samples were formed their resistance over time whileexposed to 80% relative humidity and 85° C. was tested.

As shown in this FIG. 1, the initial resistance of both samples wasvirtually identical at about 2 milliohms. While the resistance in thevapor blasted sample remained relatively constant over time, the samplebonded without micro-roughening the connection surface of the TMB had anlarge increase in resistance over time. The sample treated with thevapor blast had a approximately 10× improvement in bond resistancestability compared to the untreated sample.

Although only a few embodiments of the present invention have beendescribed above, it should be appreciated that many modifications can bemade without departing from the spirit and scope of the invention. Forexample, although aluminum has been described above as the metal for theTMB, it should be appreciated that the present invention can be used forbonding any type of electrically conductive metal used for thick metalbacks. All such modifications are intended to be included within thescope of the present invention, which is to be limited only by thefollowing claims.

What is claimed is:
 1. A method for improving the electrical propertiesof circuit board assemblies comprising the steps of: a) vapor-gritblasting with a spray pressure of at least 65 psi and no more than 150psi said connection surface to micro-roughen said connection surface; b)protecting a ground plane of a circuit board from oxidation; c)dispensing an electrically conductive adhesive on one or both of saidconnection surface and said ground plane; d) joining under pressure saidground plane and said connection surface in a clamping fixture to form acircuit board assembly; and e) curing said circuit board assembly toform a stable bond between said connection surface, said adhesive andsaid ground plane.
 2. The method of claim 1 wherein said spray pressureis about 90 psi.
 3. The method of claim 1 wherein said vapor-gritblasting is an aqueous slurry.
 4. The method of claim 3 wherein saidaqueous slurry is a fine grit abrasive suspended in an aqueous medium.5. The method of claim 1 wherein said curing is conducted at atemperature of at least 130° C. and no more than 165° C.
 6. The methodof claim 5 further comprising a curing time of at least 25 minutes andno more than 55 minutes.
 7. The method of claim 6 further comprising atemperature ramp rate of at least 10° C. per minute and no more than 12°C. per minute.
 8. The method of claim 1 further comprising inserting acompliant pad into said fixture wherein said compliant pad is in contactwith said circuit board.
 9. The method of claim 8 wherein said compliantpad is a rubber pad.
 10. The method of claim 1 wherein said clampingfixture is an enclosure.
 11. The method of claim 10 further comprisingthe step of evacuating air trapped within said bond.
 12. The method ofclaim 11 wherein said evacuating is performed by a vacuum.
 13. Themethod of claim 1 wherein said metal substrate is aluminum.
 14. Themethod of claim 1 wherein said ground plane is tin.
 15. The method ofclaim 14 further comprising the step of treating said ground plane withan oxide inhibitor before joining ground plane with said connectionsurface.
 16. The method of claim 1 wherein said connection surface has arelatively uniform roughness of about 30^(μ) in to about 70^(μ) in aftervapor-blasting.
 17. A method for improving the electrical properties ofcircuit board assemblies comprising the steps of: a) vapor grit blastingsaid connection surface to micro-roughen said connection surface; b)protecting a ground plane of a circuit board from oxidation; c)dispensing an electrically conductive adhesive on one or both of saidconnection surface and said ground plan; d) joining under pressure saidground plane and said connection surface in a clamping fixture to form acircuit board assembly; and e) curing said circuit board assembly at atemperature of at least 130° C. and no more than 165° C. for at least 25minutes and no more than 55 minutes at a temperature ramp rate of atleast 10° C. per minute and no more than 12° C. per minute to form astable bond between said connection surface, said adhesive and saidground plane.
 18. The method of claim 17 wherein said vapor-gritblasting is applied with a spray pressure of at least 65 psi and no morethan 150 psi.
 19. The method of claim 18 wherein said spray pressure isabout 90 psi.
 20. The method of claim 17 wherein said vapor-gritblasting is an aqueous slurry.
 21. The method of claim 20 wherein saidaqueous slurry is a fine grit abrasive suspended in an aqueous medium.22. The method of claim 17 further comprising inserting a compliant padinto said fixture wherein said compliant pad is in contact with saidcircuit board.
 23. The method of claim 22 wherein said compliant pad isa rubber pad.
 24. The method of claim 17 wherein said clamping fixtureis an enclosure.
 25. The method of claim 24 further comprising the stepof evacuating air trapped within said bond.
 26. The method of claim 25wherein said evacuating is performed by a vacuum.
 27. The method ofclaim 17 wherein said metal substrate is aluminum.
 28. The method ofclaim 17 wherein said ground plane is tin.
 29. The method of claim 28further comprising the step of treating said ground plane with an oxideinhibitor before joining ground plane with said connection surface. 30.The method of claim 17 wherein said connection surface has a relativelyuniform roughness of about 30^(μ) in to about 70^(μ) in aftervapor-blasting.